1. Field of the Invention
The present invention relates to a chemical mechanical polishing apparatus which is used to flatten semiconductor substrate surfaces and the like during the processing thereof
2. Description of Related Art
Recently, the increasing miniaturization and difference in levels of semiconductor devices has been striking. In order to form a minute pattern, the wavelength of the stepper light source used in the photolithographic process has been reduced in order to make smaller the horizontal resolution R shown in Formula (1), and the NA value (numerical aperture) has been made larger.
As a result, the depth of focus (DOF) during exposure becomes shallower, as shown in Formula (2), and the focus margin becomes smaller. Furthermore, when the unevenness on the surface becomes severe, the light reflected from the surface is reflected at a slanted angle during exposure, so that the exposure pattern becomes disordered. As a result, it is necessary to completely flatten such surfaces in the manufacturing process of semiconductor devices having minute patterns. EQU R=K.sub.1 .multidot.L/NA (1) EQU DOF=K.sub.2 .multidot.L/(NA).sup.2 ( 2)
Here, L indicates the wavelength of the light source, while NA indicates the numerical aperture of the optical system.
A chemical mechanical polishing method (CMP: chemical mechanical polishing) has been developed as flattening technology for semiconductor substrate surfaces in the manufacturing processes of semiconductor devices which are minute and have a large difference in levels. These chemical mechanical polishing methods can be employed, for example, in the flattening of poly Si embedded in trenches, interlayer insulating films in the case of multilevel wiring or the like, and metallic wiring in the case of aluminum alloy thin films or the like.
A schematic view of a conventional chemical mechanical polishing apparatus is shown in FIG. 8.
A Si wafer 60 comprising the substrate to be polished, having formed thereon interlaying insulating films, has the surface thereof affixed to a rotatable support stand 61 by means of suction or the like. Commonly, a cushioning material such as a rear surface pad or the like is attached between the support stand 61 and the wafer 60 which is to be polished; this rear surface pad applies a uniform pressure to wafer 60.
While rotating support stand 61, the surface of wafer 60 is pushed against stage 63, to the surface of which is affixed a polishing pad 64. Stage 63 is also made to rotate. The pressure applied is commonly within a range of 0.3-0.6 kg/cm.sup.2.
During polishing, a polishing slurry in which inorganic microgranules, such as silica or the like are dispersed, is dripped onto the surface of the polishing pad 64. A plurality of narrow groves are provided in the circumferential direction in the surface of stage 63, so that the polishing slurry might effectively flow into the interface between the surface of wafer 60 and the surface of polishing pad 64. The microgranules in the slurry promote polishing by means of mechanical action with respect to the wafer surface. In this method, wafer 60 rotates and revolves and polishing pad 64 also rotates, and the diameter of polishing pad 64 is larger than that of wafer 60.
However, the conventional chemical mechanical polishing apparatus described above proved to have the following problems.
(1) Presently, an even greater degree of flatness (+/-0.1 mm or less) is required; however, it is impossible to fulfill such requirements with the conventional apparatus described above.
(2) The uniformity within the surface is poor (at best, on the level of +/-10%).
(3) An extremely large amount of polishing slurry is employed.
(4) The polishing slurry does not penetrate uniformly to the interface, and bonding is likely to occur.
(5) When wafers having a large diameter are polished, it is necessary to employ large polishing plates, and as a result, the overall size of the apparatus becomes extremely large.
In order to flatten a surface by polishing, the polishing slurry must be uniformly supplied to the interface between the surface which is to be polished and the surface of the polishing pad, and the relative motion between the surface of the material to polished and the surface of the polishing pad must be such that the movement of the microgranules in the polishing slurry is uniform in all directions in the vicinity of all surfaces. This has not been realized in the case of apparatuses in which the surface of the polishing pad is larger than the surface of the material to be polished.
The views stated above, including the problems present in the conventional methods, are completely novel views arrived at by the present inventors.
On the other hand, a polishing apparatus has been disclosed (Japanese Patent Application, First Publication No. Hei 1-170556) in which an epicyclic gear mechanism is provided on the polishing head, the polishing pads can be made to revolve and rotate by means of the epicyclic gear, and the front surface of the polishing pads is caused to rub against the surface of the material to be polished; furthermore, a transverse slide mechanism and a longitudinal slide mechanism are provided so that the polishing head may be caused to move in a zigzag fashion relative to the material to be polished.
This polishing apparatus is capable of minutely and symmetrically finishing a comparatively large sized flat surface to a non-streaked mirrored surface, and is employed in the polishing of stainless steel plates.
However, it is unclear whether the degree of flatness of +/-0.1 mm described above can be achieved using this apparatus. In the case of semiconductor wafers, the surface does not comprise a uniform material; interlayer insulating films such as BPSG or the like, Al alloy, polysilicon, and the like may be present. When the present inventors assayed this experimentally, when semiconductor wafers were polished using this apparatus, the degree of flatness described above was not achieved, and furthermore, there were problems in that in comparison with the central portion, the outer circumferential portion was either insufficiently polished or excessively polished.